Painted-black water barrels along the north wall are the most effective greenhouse thermal mass, and 200 liters of water per square meter of glazing is a good target. That much water stores about 8.4 MJ for a 10 K swing, roughly 3.2 times the idealized overnight heat loss through the same area of single glazing, so it can carry a greenhouse through a night if heat can reach the plants.
Thermal mass in a greenhouse is dense material, usually water or stone, that absorbs surplus daytime heat and releases it slowly after dark.
The Arithmetic Behind The 200 Litres Rule Shows Both Why It Works And Where It Stops
A greenhouse without thermal mass overheats during the day and freezes at night. Water is the best mass per volume because its specific heat is high: about 4,186 J per kilogram per kelvin (a defined physical constant, recalled). One liter has a mass of one kilogram, so 200 L is 200 kg. The sum below turns that rule of thumb into energy, then compares it with the heat the glazing can lose in a night. The working uses physical constants and a textbook heat-loss coefficient, not a measured tolerance; no empirical failure threshold is claimed, and the limits at the end say where the idealization breaks.
Other options store less per volume. Stone or brick walls and dark gravel or brick floors all work, but a cubic meter of masonry stores roughly half what a cubic meter of water does (masonry volumetric heat capacity taken as about 2 MJ per cubic meter per kelvin, recalled from standard tables). Water also convects internally and spreads its heat evenly, which stone does not.
What You Need
- Water barrels — salvaged drums or casks, with sealed lids, painted black on the sun-facing side
- Placement along the north wall, so they do not shade the crop and absorb sun that reaches the back of the house
- A thermometer at plant height and one in the water, to check the sum against reality
- Optionally stone or brick for the north wall or floor
Method: Size And Place The Mass
Step 1: Measure the glazed area. Add up the area of glass or film, in square meters.
Step 2: Apply the rule. Multiply the glazing area by 200 L. A greenhouse with 20 m² of glazing gets 20 × 200 = 4,000 L of water.
Step 3: Place the barrels so the sun reaches them. Black paint on the barrels along the north wall, with the sun striking them directly, maximizes the heat gained. Do not stack them so tall they shade the plants.
Step 4: Compare day and night temperatures. Read the minimum thermometer each morning. If the water drops more than the swing you assumed, add water or insulation.
Worked Example
Per square meter of glazing.
Stored energy. Mass m = 200 kg. Specific heat c = 4,186 J/(kg·K). Allowed swing ΔT = 10 K (assumed: barrels cool from 25 °C to 15 °C overnight). Energy = m × c × ΔT = 200 × 4,186 × 10 = 8,372,000 J = 8.37 MJ (about 8.4 MJ), which is 8.37 / 3.6 = 2.33 kWh.
Night loss through the glazing. Heat-loss coefficient U for single glass or single film: take 6 W/(m²·K), the upper end of the roughly 5-6 range quoted for single glazing in standard building-physics tables (recalled, not measured here). Assume a mean inside-to-outside difference of 10 K over a 12 h night = 43,200 s. Loss = U × ΔT × t = 6 × 10 × 43,200 = 2,592,000 J = 2.59 MJ per m² of glazing.
Comparison. 8.37 / 2.59 = 3.23. The 200 kg could cover the idealized night loss about 3.2 times over.
What that does to the water. If the barrels alone supplied the whole 2.59 MJ, their temperature would fall by 2,592,000 / (200 × 4,186) = 3.1 K over the night.
Equivalent masonry volume. At about 2.1 MJ/(m³·K) for stone (recalled) and the same 10 K swing, 8.37 MJ needs 8.37 / (2.1 × 10) = 0.40 m³ of stone, against 0.2 m³ of water.
Limits Of The Calculation
- Capacity is not delivery. The mass only heats the air while it is warmer than the air, and its heat spreads by convection and radiation, not instantly. A barrel in the corner does little for a bench across the house.
- The loss figure is idealized. Wind, air leaks, cold ground, and clear-sky radiation to the night sky all add to the conducted loss, so real losses exceed the sum above, especially on still, clear nights, which are also the coldest.
- The swing is an assumption. If the barrels start the evening at 18 °C rather than 25 °C, the useful swing shrinks.
- A single 10 K swing is not a freeze guarantee. It measures energy, not crop survival; check minimum temperatures against the hardiness of what you grow.
Common Mistakes
- Dark paint on only the shaded side — the barrels never reach the temperature the sum assumes.
- Barrels shading the crop — you trade yield for heat.
- Counting stored energy as delivered heat — see the limits above.
Safety
Weight and water
A 200-litre barrel full of water weighs about 200 kg and its weight is 200 × 9.81 = 1,962 N. Set barrels on firm, level ground or a solid base, not on a raised bench or plank, and chock them so they cannot roll.
What’s Next
- Learn next: Ventilation & Overheating Prevention — mass only stores what the vents do not throw away
- Combine with: Thermal Mass Principles — the same physics for buildings